Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

643
FXUS63 KLOT 150714
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
214 AM CDT Wed Jul 15 2026

.KEY MESSAGES...

- Hot conditions will continue through the week with high
  temperatures generally in the upper 80s to 90s and overnight
  lows in the 70s.

- A period of reduced visibility due to smoke may occur on
  Thursday generally along and northeast of Interstate 90.

- A few storms appear possible on Thursday, with increasing
  storm chances Friday and then again late Saturday afternoon
  into Saturday evening.

&&

.DISCUSSION...
Issued at 214 AM CDT Wed Jul 15 2026

Another hot day is on tap as an elongated, nearly 600 dam 500 mb
high remains directly overhead. Little change to column
thicknesses today should result in similar high temperatures to
Tuesday. The HRRR/RAP/RRFS continue to be greatly overmixed and
too dry during the afternoon, while the NAM/NAMNest/NSSL WRF
remain too moist. A combination (HREF) performed respectably
well on Tuesday, and have largely followed this output for
today, which results in dewpoints at or just a smidge higher
than Tuesday. One slight wildcard today is the presence of some
upper-level smoke (which has taken a circuitous route from
southern Canada, across New England, and then back across the
Midwest). Progged upper-level smoke concentrations are a bit
higher than they were on Tuesday, but at this point, don`t think
this will be enough to result in meaningful insolation
reductions today. The net result is peak heat indices of 100 to
locally 105 degrees today away from the lake. Similar to the
past few days, the lake cooling footprint will be minimal and
tied to the immediate lakeside areas.

Tonight into Thursday morning, low-level wildfire smoke is
ubiquitously progged to surge southward out of Wisconsin and
into northeast Illinois and parts of northwest Indiana. HRRR and
RRFS smoke output suggest the highest near-surface
concentrations will be tied to areas near and behind the
previous evening`s lake breeze, where winds will be light east
to southeasterly and a significant subsidence inversion will
allow smoke to rapidly build to the surface. Forecast
concentrations resemble the June 27, 2023 event, so if guidance
is correct, there would likely be notable visibility reductions
before deepening PBL circulations help churn up and disperse
smoke (to some degree) during the afternoon. At this time, the
greatest concentrations are forecast roughly north and east of a
Rochelle to Valparaiso line, but will note a lot of this hinges
on the lake breeze passage this afternoon and evening and the
general smoke evolution upstream today.

Thursday afternoon`s forecast gets complicated a bit by the
prospect for surface smoke and reduced insolation. Additionally,
seeing a decent signal in the guidance for winds to remain light
easterly/northeasterly through the morning, coupled with a more
robust lake breeze surge midday into the afternoon. All of these
could help notably hold temperatures down across NE Illinois and
near the lake. Since this is not a high confidence scenario at
this time, have only slightly nudged highs down roughly northeast
of I-90 into the mid 80s. Less smoke or winds turning offshore
during the morning could easily push temperatures back into the
low 90s.

Given all of this, and with peak heat indices only touching 100
degrees around Chicago on Monday, am less concerned about
flirting with the 3-day Extreme Heat Warning threshold for Cook
County. The continued general heat messaging continues to appear
appropriate for this stretch.

Low to mid-level flow will begin to turn out of the south during
the afternoon as a broad cyclonic gyre that`s been present
across the ArkLaMiss gradually advances northward through
Thursday afternoon and evening. This will push increasing
boundary layer moisture northward, and some very modest upper-
level jet divergence is forecast to develop as an embedded vort
max meanders across northern Missouri. Forcing obviously won`t
be robust, and soundings still look pretty marginal from a
thermodynamic perspective, but this may be enough to facilitate
some isolated shower and thunderstorm development during the
afternoon, particularly in the vicinity of the lake breeze. Cell
motions would be slow, but the lack of stronger shear suggests
any storms will remain pulse-like, reducing the residence time
of any heavier rainfall rates.

For Thursday night into Friday morning, Environment Canada
smoke output suggests another surge of smoke could occur in the
region before deeper southwesterly flow gets established over
the region on Friday. Have not continued a smoke mention during
this time frame at this juncture, but may need to consider this
in future forecast packages.

On Friday afternoon, a series of upper-level perturbations are
forecast to drift across the region. This should foster higher
coverage of diurnal showers and thunderstorms. Cell motions will
once again likely be quite slow, and given higher coverage and
forcing mechanisms, may result in somewhat more of a locally
heavy rainfall threat.

On Saturday, the guidance trend has been towards a slower
progression of a robust shortwave embedded within the broad
cyclonic flow to our north. While this feature will eventually
shove a cold front through the region, it`s possible this
doesn`t occur until very late in the afternoon or into the
evening. Ahead of the front, heat indices could push back into
the 100 to 105 degree range. Additional thunderstorm chances
will accompany the front late Saturday afternoon and evening.
With increased deep-layer flow, will need to keep an eye on the
potential for some increased storm organization but at this
time, the severe threat still appears generally low across the
area.

Sunday now looks like it may remain dry as high pressure builds
back across the region, with temperatures falling back into the
80s. Another fast-moving disturbance will bring the next chance
for thunderstorms to the region during the Monday/Monday night
timeframe. This wave is forecast to be accompanied by
significantly more flow throughout the column, so one to keep an
eye on for a possible stronger storm threat, although
uncertainties at this range remain substantial.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1226 AM CDT Wed Jul 15 2026

Key Aviation Messages for the current TAF period:

- A lake breeze is expected to switch wind directions at Chicago
  terminals to the east this afternoon.

- Smoke from Canadian wildfires will slowly move into the region
  at the end of the TAF period. Low confidence in vis trends and
  timing.

VFR conditions are expected through the TAF period. Light
westerly winds will remain through the day for terminals away
from lake Michigan. There is a little better confidence than
recently of a lake breeze moving through this afternoon
switching wind directions to east. However, winds should remain
below 10 knots.

Wildfires are ongoing in Ontario, Canada. The upper level
pattern has pushed most of the smoke over to the eastern
seaboard, but that high level smoke will slowly start to filter
into the region late afternoon and evening giving the sky a
hazy look. Overnight, the higher concentration of smoke in
southern Ontario is expected to move southward toward northern
Illinois by Thursday morning. There is a better chance for lower
vis with this smoke, but there is uncertainty on the exact
timing of when it arrives and how low vis will go. For now,
added VFR FU to the end of the RFD TAF and MVFR to ORD as
confidence in getting south of there by 12Z is still too low to
add to DPA/MDW/GYY.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT tonight for ILZ005-ILZ006-
     ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...Air Quality Alert until midnight CDT /1 AM EDT/ tonight for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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